In LTE-Advanced systems, relaying will be used to enhance coverage and increase data rate at cell borders without increasing the number of base stations.
Type-II relay, also named L2 relay, is considered to be one of the potential technology components of LTE-Advanced. A type-II relay node is an in-band relaying node characterized by the following [1]:                It does not have a separate Physical Cell ID and thus would not create any new cells.        It is transparent to Rel-8 UEs (User Equipments); a Rel-8 UE is not aware of the presence of a type-II relay node.        It can transmit PDSCH (Physical Downlink Shared Channel).        It does not transmit CRS (Common Reference Signal) and PDCCH (physical downlink control channel).        
As type-II relay does not have PDCCH, the downlink control coverage can not be extended. The major advantage of type-II relays is transmission rate improvement. However, the system design should be revisited carefully considering the new additional node and its specific characteristics. One of the most important aspects is link adaptation which is crucial to the system throughput as well as user experience and quality of services.
Type-II Relay Operation
Due to the transparency of type-II relay, the UE is not aware of the presence of the relay although it is receiving help on the data channel. In this invention, synchronous non-adaptive HARQ (Hybrid Automatic Repeat reQuest) is assumed in order to simplify the design. More complex schemes can be considered as well. However, the potential benefits have to be traded off against flexibility and overhead of HARQ timelines. A relay-assisted downlink transmission consists of two phases as shown in FIG. 1:                First transmission: In the first phase, the eNodeB will transmit data to the UE and the relay will sniff the data at the same time. Both the UE and the relay will try to decode the data. However, in order to exploit gain of the relay-assisted transmission, there is a high probability that the UE can not decode the packet in this first transmission attempt so that re-transmission happens in the second phase.        Re-transmission: In the second phase, the eNodeB and the helping relay will transmit the data to the UE. As a result, the UE receives boosted signal strength by means of over-the-air combining of the direct link which is from the eNodeB to the UE (eNodeB→UE) and the access link which is from the RN (Relay Node) to the UE (RN→UE). In addition, the soft bit information from the first transmission can also be exploited via chase combining or incremental redundancy. This is a further improvement at the cost of additional memories and computation complexity.        
It should be noted that for synchronous non-adaptive HARQ transmission, a common modulation and coding scheme (MCS) is selected for the first transmission and re-transmissions. Moreover, this MCS scheme is selected to fit for the quality of the combined channel of the direct link and the access link. The behind reason is that relay-assisted transmission is expected to improve the overall performance than direct transmission. The combined channel can be a sum of the channel gain of direct link and access link (or even better if the first the transmission is also exploited). In typical deployment of type-II relays, the channel quality of the access link is much better than the direct link. As a result, the combined link is much better than the direct link.
Outer-Loop Link Adaptation
Link adaptation is adopted in most of the modern wireless communication systems. Crucial to the working of fast link adaptation is the timely reporting of channel conditions that is fed back from the receiver to the transmitter. A generic term “channel quality indicator” (CQI) is often used to refer to any of such feedback, comprising SNR (Signal to Noise Ratio) and SINR (Signal to Interference Plus Noise Ratio), etc. In a LTE system, common reference signals (CRS) is used to do CQI measurement and different kinds of channel quality indicator can be reported to support flexible scheduling methods.
However, the delays of CQI reporting (including the propagation delay and the processing delay) as well as the time varying channel conditions and interference conditions pose big challenges to link adaptation. The selected MCS based on reported CQI may not be proper for the data transmission. Aggressive MCS selection will cause high block error rate (BLER) while conservative MCS selection will result in low spectrum utilization.
An outer-loop link adaptation can be used to dynamically control the average BLER for the first transmissions based on the acknowledgement feedback (ACK/NACK) from the UE [2]. It follows the same principle as the traditional outer loop power control algorithm for dedicated channels in IS-95 and WCDMA and for HSDPA. Here we define an offset factor A. If an ACK is received for a first transmission, the offset factor A is increased by one preset step Aup, while it is decreased by one preset step Adown if a NACK is received. The modified offset factor A then provides a modified CQI which is used as a basis for selecting MCS. The ratio between Aup and Adown determines the average BLER that the OLLA converges to, i.e.BLER=1/(1+Adown/Aup)  (1)
For example, if the BLER target is 10%, the ratio of Adown and Aup should be 9. The configuration for the adjustment steps will have significant impact on the convergence and the stableness of the algorithm. A separate outer-loop link adaptation algorithm is maintained for each user, as users may have different CQI measurement errors and performance.